ΔvΔt
by PreludeInZ
Summary: Deceleration is a function of velocity over time.


It takes approximately two minutes to fall twenty thousand feet, though this is an estimate, and varies based on body position, wind speed, whether or not the person falling is possessed of a wing suit, and a handful of other factors.

John's aware of each and every one of these factors and, further, has up-to-the-microsecond telemetry available to calculate them, as his older brother plummets out of the sky.

Scott knows them too, but he's still wasted about ten seconds of precious time with panicked shouting.

According to John's math, with his limbs pinwheeling and his body nearer to vertical than horizontal, Scott's only got 16,328 feet and about a minute and twelve seconds. Ten seconds of yelling is a terrible, _terrible_ waste of time.

But then, John's not the one plummeting through the troposphere, so he's trying not to judge.

Especially considering his brother is plummeting through the troposphere with neither parachute nor wingsuit, and is instead possessed of a malfunctioning jet pack, shorted out by an electrical discharge from the research drone he'd been attempting to repair. On the whole, John's judgement is probably better spent on working out a solution to this rapidly (if not literally) escalating situation.

"Thunderbird One, flip over, spread out," he orders, his voice sharp and commanding in Scott's ear. Scott and Kayo are technically the only two licensed skydivers, and Scott only as a precautionary measure. "Get into neutral freefall, you need to slow down."

The only answer is a huge heave of breath, but Scott does as he's told and John's readouts track the changes and update, giving Scott another thirty-four seconds, bumping the total up to a minute-forty. Better, but still not great, though it does allow Scott to get a hold of himself to the point that he can answer, " _Shit_ , oh _fuck_ , holy _Christ_."

"I need status on your jetpack."

Scott's comm is inside his helmet, so John's spared the noise of the wind around him, but his brother still shouts over it, fills TB5's comm sphere with tightly controlled panic. "Control shortage, must've been a crack in the casing. Probably in safe mode. I think I can test burst, but the override'll kick in to prevent a straight burn, can't hover."

John's not getting _anything_ off the system display for Scott's jetpack, so this seems like a correct assessment. He pulls up auxiliary data and starts to skim through the details of the jetpack's safemode. A minute and twenty-two seconds, 13,260 feet. "Fuel status?"

"Nearly a full tank, not the problem. I can't fire for more than a half second at a time."

This tracks with the same data John's skimming over and he chalks that up as a point in his favour, starts to coalesce a solution around the numbers he's got to work with.

There's a crackle of static and Scott proposes, "—hard reset, see if that brings it back online. Might be able to—"

"Negative, Thunderbird One, _do not_. Repeat, _do not reset_. No guarantee you'll get it back at all, and safemode's better than nothing."

There's a second's pause (a hundred and seventy-six feet feet ) and Scott's answer is terse, denotes a narrowly avoided error, "Copy. John, _fix this_."

"Standby." John, at least, has had enough time hit upon a potential solution. The _only_ solution, really, given the extremely limited resources he's got in play. It's nowhere near the realm of ideal, but it just might make the difference in survivability. It's going to take _math_ , but he's got that more than covered. "EOS, calculate available thrust versus rate of descent and work out how much we can slow him down."

It's possible she's already done it, or it's possible she's just that fast, because almost immediately she has an answer for him, "Half second bursts at decreasing intervals as marked will be enough to arrest his rate of descent into a survivable range."

Before John can ask, she renders this timeline for him, a clear and neatly labeled schematic that marks every interval at which his brother needs to fire his jetpack, in order to function as a sort of airbrake, and slow him down as he approaches the choppy surface of the Atlantic Ocean. She's included all the necessary calculations of thrust and drag, weight versus lift, of velocity over time and every other relevant mathematical consideration—but John doesn't have time to do anything more than take them in at a glance, and trust that they're correct. The math is his 'bird's job. Telling his brother the details of the plan as it stands is John's.

12,560 feet. A minute and twelve seconds. He engages his comm again. "TB1, you're going to fire your thrusters at specified intervals, on my mark. Half-second burst on a twelve-second interval, diminishing down to a two-second interval. You should slow down enough that you'll be able to brake in midair about thirty feet before you hit the water. Copy?"

Presented with a viable solution, Scott's voice is calmer than it has any right to be, for someone in freefall, twelve thousand feet above the surface of the Earth. "Copy, TB5. Like a parachute that only works for half a second at a time."

"Like an anti-lock air-brake."

There's a pause of a few valuable seconds before Scott asks, "Is it gonna work?"

"Yes." There's nothing else to say.

"You did the math?"

Well, yes and no. John had known what math needed to be _done_ , he could conceive of the solution to the problem, and the fact that the problem _had_ a solution, but in terms of actually inputting and completing the calculations relative to ongoing telemetry—"EOS did."

Another beat of silence, and then— "You gonna bet my life on her?"

This is neither the time nor the place for this argument, but John still takes a steadying deep breath, and keeps his tone firm and even as he answers, "You've seen me bet mine."

Apparently this answer is good enough, because Scott's next question is brusque, businesslike. "How much longer am I gonna spend in freefall?"

John checks. "Forty-five seconds. You'll fire for the first time at three and a half thousand feet, on my mark."

There's a truncated bark of laughter in response to this, "Oh, well, that's almost _leisurely_."

"You'll be fine."

"You say so, Johnny."

It's not just banter. It's an indication of complete, absolute trust, and as he watches a holographic representation of his brother, falling, John can't let himself think of anything interpretation of events except the one in which everything works as planned, even if the plan was conceived hastily, literally on the fly. Forty-five seconds is a longer span of time than anyone realizes, and so far this might just be the longest couple minutes of his life. He can't imagine what it feels like for Scott. Moment by moment of silence ticks past, and before much longer John feels the need to break it.

"So," he starts, impossibly casual, though he needs to swallow past the pressure at the back of his throat before he can continue, "— _So_ , how would you say _your_ day's going?"

His big brother's laughter is something they both need to hear, as the clock ticks down to past the half minute mark. T-minus thirty before this plan goes into action. Scott's fallen two-thirds of a mile in the twenty seconds that have passed since John proposed his solution. Now his brother's only nine thousand feet from the ground, and John feels his heart skip a beat as five digits dwindle to four.

"You know," Scott answers, sarcastic to a degree that can only be achieved when at terminal velocity, absent a parachute, " _shockingly_ , I've had better."

"For the record, _this_ is why you have a safety line. For when you fall off a research drone in the troposphere and short out your jetpack."

"There wasn't anywhere to clip it!"

"You have a jet plane!"

"Yeah, and I couldn't bring her close enough to attach a line without knocking that stupid damn drone out of the sky!"

"We could've—"

"Hey, know what? I'm not actually going to have this argument with you, right now. If I'm not dead in the next two minutes, _then_ we'll talk."

"You'll be fine."

"I'd better be."

"Feet first. Point your toes. Arms out, arch your back. In 1942 Alan Eugene Magee survived a twenty-two thousand foot fall out of a B-17 and through the glass roof of a railroad station. You've got this." Scott's not usually on the receiving end of pep talks, nor are they a particularly strong skill of John's, but it seems like the thing to say.

"That's very helpful, thanks."

John looks at his charts and his readouts and his timer and his timeline, and watches as the schematic continues to update in real time. Not for the first time he's almost overwhelmed and astounded by the level of technology he just _lives_ with—which permeates his entire life, practically defines his personality and his purpose—but which he almost takes entirely for granted. When this is over, he's going to need to take the time to deliberately wrangle with the immensity of it all, but right now, there's a countdown that needs to start. "T-minus ten 'til your first mark. Ready?"

"Oh, I was _born_ ready."

There's steel in Scott's tone, the sort that indicates that he hasn't the least intention of dying today, and that if the laws of physics have a problem with that, then that's just too damn bad.

"Eight."

"Seven."

"Six."

"Five."

"Four."

"Three."

"Two."

"One—Mark."

John doesn't have the readout from Scott's jetpack, but over the open radio channel he hears the low, guttural burst of the engine firing, and immediately his telemetry updates to reflect the change. The burst isn't quite enough to overcome 120 mph of terminal velocity, but it does slow the rate of descent, and Scott will gain about six seconds worth of time before he's back in freefall again.

So the theory is sound, and it's working in practice. And for the first time since things started going wrong, John feels a flutter of hysterical relief bubbling up. The reality that he's found a solution to the problem is the only thing that allows him to consider the possibility that he might _not_ have. If Scott had been incapacitated. If his jetpack had been rendered inoperative. If he didn't happen to be over the Atlantic Ocean. If they didn't have EOS.

It doesn't bear thinking about, and it distracts him for only the tiniest fraction of time. In holograph, he watches his brother's icon, falling towards the next calculated point, 2,671 feet.

"Mark."

It works again, like magic. It's only math, and John's perhaps overly fond of rendering complex problems into the principles of simple physics, but this is one of those rare times when he can see the shape of a miracle in the curve of a graph. In this case, a graph plotting a change in velocity over time, enough to actually, literally save his brother's life—that's nothing short of miraculous. It's _karmic_ , almost, not that John believes in karma. But that the factors that aligned to cause a catastrophe like this would also align with the factors necessary to mitigate it—it's almost beyond the pale. It's hard not to believe that Scott's going to survive this because he just _deserves_ to.

"Mark."

Beyond these single word exchanges, there's not a great deal of room for conversation, and the concentration required on both their parts is immense—moreso for Scott than for John. John's heart is hammering in his chest, though his tone can't belie even the slightest trace of anxiety, because Scott needs to trust him, absolutely and completely. Scott's biometrics display a predictably elevated heart rate and blood pressure, but likewise, there's no such betrayal in the rock steady tone of his voice, now that there's a plan that he's able to execute. Now that his survival depends on his skillset and his reaction time, Scott's got the uncanny capacity to know and trust in his own capabilities.

"Mark."

This time the half-second burst of Scott's jetpack is almost enough to overcome the rate of his fall, and he slows further still. By John's calculations, a half-second burst won't ever be enough to overcome the force of gravity pulling his brother Earthward—he won't ever be able to really _stop_ —but if they continue to push back against the rate of descent, the nearer they get to the surface of the water below, the slower he'll be going.

"Mark."

It's still nearly two thousand feet, but it's being bitten into chewable, bite-sized chunks of distance. It takes approximately twelve seconds to fall four hundred and fifty feet, and achieve terminal velocity at approximately 120/mph. Scott's no longer letting that happen, now that John's found him a rhythm of action, he's no longer falling for long enough to hit that upper limit again.

"Mark."

That throaty cough of rocket fuel is rapidly becoming John's very favourite sound in the entire world, because every time he hears it, there's a correlating little bounce of the graph projecting his brother's rate of deceleration. Each time John gets a rewarding little spike of adrenaline, the thrill that it's actually _working_ ; that it's _still working_ , that it's not just a fluke. That despite the casual day-to-day impossibility of their lives, despite the fact that catastrophe is their family's bread and butter, despite the fantastical nature of this incredibly improbable problem, it's still a problem that he and his brother have been able to _solve_.

"Mark."

The numbers have all grown so abstract that he can't actually get his head around them any longer—can't spare a moment of concentration to imagine what a thousand feet above the Atlantic Ocean actually looks like. All five of them, at times, have felt like extensions of their Thunderbirds—John's never been able to decide if this is something that he feels to a greater or a lesser degree than his brothers do. Five isn't as kinetic as the other four crafts, but she's also _more_ , somehow. His station is omnipotent and omnipresent, and right now he's just a voice for the data TB5 has provided, trusting utterly and entirely in the fact that this information is exactly what he'd asked for, and exactly what he needs. The transition from four digits down to three happens, but this time he doesn't notice it.

876 feet.

"Mark."

Afterwards they're probably going to have a fight about this. John will conveniently allow himself to forget the number of times he's been out on TB5's exterior, working without a tether and just telling himself that nothing would go wrong. He'll give his brother the same sort of hell that their father would, if he were still around to do so. _Someone_ has to. Scott will take the scolding with the appropriate grace and with due consideration of the fact that it took the pair of them working in tandem to pull this off. He'll swear never to work without a safety line again. John won't believe him, but will let it go, and will quietly remain a hypocrite himself.

651 feet.

"Mark."

There's the question of how they'll tell the others— _whether_ they'll tell the others. John's perhaps more aware of it than anyone else, just how many times the narrowness of narrow misses gets lost in translation. High above and far away, he still sees the actual numbers in greater perspective than anyone not actively involved in the situation in question. This narrow miss still has a margin of hundreds of feet to go horribly, _horribly_ wrong, but John's seen margins of inches, of _seconds_. He's seen Alan with a one in ten chance of survival, he's seen Virgil go stubbornly forward into deeper danger when he _needs_ to go back towards relative safety, he's seen Gordon play with clearances of only a few inches between him and certain death. This isn't even the first time he's watched Scott falling out of the sky. Whether or not this is a bit of news that rates a casual mention over dinner remains to be seen. " _Oh by the way, I nearly died today, could you please pass the salt?"_

544 feet.

"Mark."

If their roles were reversed, John wonders if they would pull this off. He and Scott think differently, they always have. If John were One and Scott were Five, it's entirely possible that the solution they'd found would've been altogether different—in John's position, Scott would've immediately taken remote control of a few hundred tons of steel and fire, and sent One rocketing earthward, to the rescue. John had considered the possibility only briefly, but had written it off as a backup plan almost as soon as he'd thought of it—he doesn't have the skill or the delicacy to pilot his brother's 'bird from twenty-thousand miles away, towards a target he can visualize but can't actually _see_. He'd have done it, if it had been the only way, but he wouldn't have been nearly as assured of success. Scott would've done it without a second thought.

386 feet.

"Mark."

This is either the best or the worst possible time to start thinking about all the things he admires and appreciates about his big brother. Not the least of these is the fact that Scott trusts him enough to come up with a solution save his life, and that he trusts Scott enough to be able to pull it off. Whatever else can be said about Scott—and at the height of frustration, in the heat of many heated moments, John's definitely said a _lot_ —it's an undeniable truth that he's got nerves of steel.

316 feet.

"Mark."

John couldn't do it. It's a poorly kept secret that he's developed a very particular fear, since taking up the role of Thunderbird Five, not of heights, but of _falling_. The single word that best sums up his relationship with gravity is "complicated", and on the ground, he tends to be plagued by nausea and vertigo, and a persistent clumsiness that lands him flat on his ass more often than not. But his bruised tailbone and ruffled dignity are just the most visible symptoms of a quiet phobia, that keeps him far away from edges and has him disdain even the loft above the villa's main lounge.

243 feet.

"Mark."

Once, too long ago to remember how old either of them had been, they'd lived in a house where one of the bedrooms had blue wallpaper and white carpeting, and they'd played for what felt like hours, jumping off the bed onto a pile of pillows and sheets and blankets, pretending that the carpet was the tops of the clouds and the blue of the walls was the blue of the sky. John wonders for the briefest moment why he's remembered that now, but of course it's obvious.

192 feet.

"Mark."

They've hit the range at which high-divers set records, this is completely, imminently survivable. This is the sort of thing that Gordon would hear about, and wouldn't _know_ how to do, but would squint at speculatively and be certain he could work up to it. If they tell him about it, in all probability, he'll start looking at the island's high, craggy cliffs with a new daredevil scheme in mind. That's a reason not to tell anyone.

115 feet.

"Mark."

This is _going_ to work, this _is_ working, this _has_ to work.

78 feet.

"Mark."

This time, according to telemetry, the force of Scott's jetpack against his downward momentum is enough to bring him almost to a full stop, and for just a fraction of a second, his speed dips towards zero before the pack cuts out and he's falling again. But this is perfect. This is the second best possible outcome, and the math updates to reflect that, that even if Scott doesn't get off one last shot, right at the 33 foot mark, he'll still probably be fine.

Probably.

33 feet, the last calculated target, and the height of an Olympic high dive.

"Mark."

One last burst, and this time Scott actually gains a few feet of height before he starts to fall again, and then three more seconds, and then it's over. John doesn't hear the splash, but he sees his brother's altitude hit zero, and while there's a sudden jump in all his vitals as he hits the water, none of it ticks up into the red.

The silence that follows is expected. Scott has to bob back up to the surface and probably get his breath back. The way the same silence stretches out over a long, interminable ten, twelve, fifteen seconds is permissible, even. The way John's gaze stays fixed on Scott's vitals is understandable, and the way he suddenly starts to doubt all of that omnipotent, omnipresent tech and telemetry is just a function of the fact that reality's suddenly caught up to him, and he's realizing that his brother's just _fallen out of the sky_ , over three miles' worth of distance, without a parachute or a wingsuit, or anything but a malfunctioning jetpack and a wildly improbable solution to an incredible, impossible problem.

It's possible that John's having a moment of his own, high above and far away from his brother, because it takes a lot longer than a moment for him to realize that his brother is insistently, almost irritably, calling his name. " _John._ Thunderbird Five, do you copy? _Please respond_. For chrissakes, Johnny, if my damn radio's gone out—"

Time syncs back up and he rubberbands back into the appropriate moment, as he catches up to his delayed reaction. A breath he hadn't realized he was holding explodes out of his lungs as he recovers the power of speech. "…here. Sorry, copy, yes, sorry, _here_. I'm here. Sorry. Scott? Holy _shit_ though. Scott? Scotty, are you—"

"Fine, J, take a breath." There's a pause and a hiss of radio static as Scott amends, "Mostly fine. Ow. I'm okay, I think, but _ow_ , though."

"People do that for fun," John points out, almost non-sequitur. His brain seems to have suddenly disassembled itself. "People…in 1977 a S-Serbian flight attendant was the only survivor of the midair explosion of a commercial plane over the Czech Republic and—f-fell. Fell over _thirty-three thousand_ feet, and—a-and…"

"John, _breathe_. I'm all right. Focus up a sec, okay? Can you bring my 'bird down for me? It's kinda choppy down here."

"Thunderbird One remote override engaged." But it's not John who says so, and it takes him a moment to realize that he's had nothing to do with the digital interface in front of him, replacing the metrics he's been staring at for the past two minutes—has it only been two minutes?—as TB1 comes out of standby and starts to make its graceful descent towards the surface of the Atlantic. EOS has taken over, and for the moment John knows better than to mention that to Scott.

"Are you okay?"

But once again it isn't John who's spoken, and the answer takes longer than it has any right to, considering the fact that nothing's happened to _him_. What _could've_ happened would've happened to Scott, that sudden, world-ending impact. John hasn't allowed himself to think about it before now, and now that the possibility has passed, he manages to stop himself once more. "Fine. Fine, I'm fine. Shit. _Fuck_. Why're you asking _me_? Are _you_ okay?"

Scott chuckles. "I'm bobbing around the Atlantic like an idiot who didn't have anywhere to clip his safety line, but for falling about fifteen thousand feet without a parachute, all things considered, I think I'm pretty all right."

"Holy _shit_ , though."

Scott laughs, now, though John catches the burble of faint hysteria at the end of it, as the adrenaline starts to bleed off. "Christ, Johnny, with the language. Is this all it takes to put a dent in your professionalism? One little three mile fall?"

"You _did_ that."

"A little bit."

" _Jesus_."

"Was substantially less helpful than you were, actually."

John shakes his head, even though Scott can't see it. His contribution to the entire thing still seems abstract, almost meaningless. All he'd done was been a voice on the comm. "EOS did the math. You did all the work. I just did my job."

"Still couldn't have done it without you." There's a pause, long enough that John feels the pull of gravity in the space of the silence, before his big brother says, "Thanks, John."

And there's no other possible answer but, "Any time, Scott."


End file.
